Microbial use of organic and inorganic substrates in metabolic processes can cause detectable changes in measurable parameters such as pH and oxygen utilization rates. The metabolism of certain organic and inorganic nutrients results in the production of various products including, but not limited to, energy, water (H.sub.2 O), carbon dioxide (CO.sub.2), hydroxyl ions (OH.sup.-), hydrogen ions (H.sup.+), and the like. As the environmental supply of exogenous organic and inorganic substrates changes, overall metabolic rates and/or patterns can change in response. Such changes can affect other measurable parameters whose values are influenced and dependent upon the presence of and/or rate at which the substrates are metabolized. The microbially mediated reactions presented below illustrate examples of how the metabolism of organic and inorganic substrates can affect measurable, dependent parameters like oxygen utilization and extracellular pH: EQU CH.sub.3 COOH+2O.sub.2 .fwdarw.2CO.sub.2 +2H.sub.2 O (1) EQU NH.sub.4.sup.+ +2O.sub.2 .fwdarw.NO.sub.3.sup.- +2H.sup.+ +H.sub.2 O (2)
Reaction (1) describes the aerobic degradation of an organic carbonaceous substrate (CH.sub.3 COOH) and illustrates the direct quantitative relationship between substrate consumption and oxygen utilization. Reaction (2) describes the biologically mediated conversion of ammonia (NH.sub.4.sup.+), an inorganic substrate, to nitrate (NO.sub.3.sup.-) in a process commonly referred to as "nitrification". This reaction illustrates the direct quantitative relationship between ammonia conversion and both oxygen utilization and the production of hydrogen ions (H.sup.+). The production of these hydrogen ions (H.sup.+) can ultimately affect the pH of the extracellular media.
Furthermore, when CO.sub.2 gas from aerobic degradation of organic substrate(s) is dissolved in water, carbonic acid, a relatively weak acid, is formed according to Reaction (3): EQU CO.sub.2 +H.sub.2 O.revreaction.H.sub.2 CO.sub.3 ( 3)
Carbonic acid dissociates in water to form carbonic and bicarbonic ions according to Reactions (4) and (5): EQU H.sub.2 CO.sub.3 .revreaction.HCO.sub.3.sup.- +H.sup.+ ( 4) EQU HCO.sub.3.sup.- .revreaction.H.sup.+ +CO.sub.3.sup.- ( 5)
This carbonic acid "system" provides a buffering effect on the pH of the extracellular media.
Because of the quantitative nature of these biochemical reactions, the abundance and/or availability of reactants, i.e., available exogenous inorganic and organic substrates, can affect the magnitude of changes in other parameters. For example, if the nitrification reaction (2) described above is a predominant reaction within a microbial culture, the production of hydrogen ions (H.sup.+) from the nitrification process would be expected to decrease markedly upon the exhaustion of readily usable ammonia (NH.sub.4.sup.+) below some metabolically critical level. Consequently, the activity of hydrogen ions in solution, i.e., pH, would also be expected to change.
Similarly, the oxygen utilization of a microbial culture would be expected to be higher in a condition in which exogenous organic substrates were readily available and plentiful than in a condition where these substrates were depleted below some metabolically significant level. In both of these examples, the measurable rate of change in pH, sometimes hereinafter referred to as "pH variation rate" or "pHPR," and oxygen utilization, sometimes hereinafter referred to as "biological oxygen consumption rate" or "BOCR," would be directly affected by the rate of substrate metabolism over time. Thus, assuming that changes in pH and oxygen consumption in a medium result from microbial metabolic activity alone, pHPR and BOCR could theoretically be used to signal metabolically significant transition points in a microbiological process.
U.S. Pat. No. 5,013,442 to Davis et al discloses a method of aerobic wastewater treatment which utilizes alkalinity as a control parameter. However, "alkalinity" is defined in Davis et al as the ability to buffer acids determined by titrating with sulfuric acid to a select end point of 4.5 pH. Thus, alkalinity as defined in Davis et al is clearly different from pH. Further, Davis et al fails to disclose the use of pHPR and/or BOCR together as control parameters for a microbiological process and also fails to teach any means for blocking non-microbial effects on pH and D.O.
Analytical methods to determine maximum utilization rates of ammonia, nitrate and oxygen are disclosed in the article "Characterization of Functional Microorganism Groups and Substrate in Activity Sludge and Wastewater by AUR, NUR and OUR," Wat. Sci. Tech., Vol. 25, No. 6, pp. 43-57 (1992). However, there is no disclosure of a method of control which utilizes pHPR and BOCR together as control parameters for a microbiological process or any means for inhibiting non-microbial effects on pH and D.O.
A model of the nitrifying activated sludge process is used in the article "Effects of Oxygen Transport Limitation on Nitrification in the Activated Sludge Process," Research Journal WPCF, Vol. 63, No. 3, pp. 208-19 (1991) to investigate the effects of mass-transport resistance and heterotrophic/nitrifier competition on the apparent relationship between dissolved oxygen concentration and nitrification. However, there is no suggestion that pHPR and BOCR be used together as control parameters for a microbiological process or of any means for inhibiting non-microbial effects on pH and D.O.
There remains a need for a practical and convenient method, usable in virtually any microbiological process in a liquid medium, for monitoring transitions in microbial metabolic behavior and controlling a microbiological process according to those transitions.